Baltimore Orioles third baseman Manny Machado may not be ready for Opening Day after experiencing a setback in his surgically repaired knee, manager Buck Showalter said.

Machado has not appeared in a game this spring and was scheduled to undergo an exam Tuesday on his knee. But the 21-year-old has been hampered by scar tissue in the knee and has not resumed running, prompting the Orioles to postpone his medical appointment.

"There's no sense in doing [the appointment] until he can really show them what he can do running," Showalter told reporters Saturday, according to MLB.com. "So that's got to get cleared up first, where he's back running again. And then he can completely clear him again."

Showalter acknowledged that Machado will be hard-pressed to be ready for the Orioles' regular-season opener on March 31 against the Boston Red Sox.

"I wouldn't say very, very difficult," Showalter said. "In four days we can get Manny more plate appearances than guys over here are going to break camp with. ... I'm not there yet. I'm not going to throw that wet blanket over that yet. I know Manny's not."

Machado injured his knee on Sept. 24 running the bases at Tampa Bay. He's changed the way he runs to avoid future injuries, but Showalter emphasized that Machado's knee is fine structurally.

"The way I understand it, it's scar tissue that flared up and got a little sore, and we decided not to push it," Showalter said. "Which is what we talked about all along -- we are not going to push it to the point where it ends up being something.